Production of hydrocarbon liquid fuels from coal may be accomplished, in part, by the Fischer-Tropsch catalyzed synthesis of hydrocarbons from synthesis gas (CO+H.sub.2) produced by coal gasification. The Fischer-Tropsch synthesis (or conversion), however, is unselective in nature, producing molecules with carbon numbers typically in the range 1-40 in proportions controlled by process kinetics. Process efficiency would be enhanced by selective conversion of the synthesis gas to relatively narrow carbon number range hydrocarbons within the general range of C.sub.4 -C.sub.25 hydrocarbons.
Fischer-Tropsch conversion of synthesis gas has heretofore been conducted both in fixed bed, gas-solid reactors, gas entrained fluidized bed reactors and in slurry phase reactors. The former is described by H. H. Storch et al in "The Fischer-Tropsch and Related Syntheses," Wiley, 1951, while the latter is described by Kolbel et al (Catalysis Review Science and Engineering, 1980, 21, 225), Poutsma (ORNL-5635, 1980, "Assessment of Advanced Process Concepts for the Liquefaction of Low H.sub.2 /CO Ratio Synthesis Gas" and Deckwer et al (Industrial Engineering Chemical Process Design Developments, 1982, Volume 21, pages 222 and 231). The latter references in particular indicate the potential incentive for using high CO/H.sub.2 ratio synthesis gas (sometimes referred to as "syngas") in liquid phase slurry reactors. More recently, Satterfield et al (Industrial Engineering Chemical Process Design Developments, 1982, Volume 21, page 465) described literature on product distribution in Fischer-Tropsch syntheses, particularly slurry reactors using iron catalysts. All of these analyses indicate that the product selectivity in such syntheses follows a predicted Schulz-Flory distribution, characterized by a chain growth probability factor (alpha), and that any reported deviations in publications concerning the Fischer-Tropsch process are probably due to experimental artifacts.
The maximum fuel product fractions predictable from the Schulz-Flory distribution, together with an indication of methane and C.sub.26+ fraction are as follows:
Schulz-Flory Maxima
______________________________________ C.sub.1 C.sub.5-11 C.sub.9-25 C.sub.26+ Alpha wt. % wt. % wt. % wt. % ______________________________________ Gasoline Range 0.76 5.8 47.6 31.8 0.7 Diesel Range 0.88 1.4 31.9 54.1 12.9 ______________________________________
In general, prior publications indicate that most Fischer-Tropsch (FT) products adhere to the Schulz-Flory (SF) distribution, regardless of catalyst type. Satterfield et al specifically concluded that the distribution generally held for iron based FT catalyst with values of alpha ranging from 0.55 to 0.94. Short term success in selective syngas conversion, by utilization of a proposed metal particle size effect or by use of Co.sub.2 (CO).sub.8 on alumina or Ruthenium in zeolites of selective pore size distribution, for example, has been reported by Nijs et al (Journal of Catalysis, 1980, Volume 65, page 328 and Blanchard, J. C. S. Chem. Comm., 1979, 605. However, none of these previous attempts has produced a stable selectivity over a period of time, the observed selectivity in each case disappearing over a period of hours and reverting to the expected Schultz-Flory distribution.
In addition to the foregoing, the following patents have been considered specifically in regard to the patentability of the present invention.
U.S. Pat. No. 4,219,447-Wheelock describes a hydrocarbon reforming catalyst and the process for its preparation. The catalyst is comprised of a Group VIII metal, preferably a Group VIII noble metal, deposited on an alumina support previously treated with a solution of Group IV B compound (a compound of titanium, zirconium, or hafnium, such as an alkoxide according to the specification of this patent); for example, the alumina catalyst base is contacted with a hydrocarbon solution of zirconium propoxide. In the preparation of this catalyst according to the Wheelock patent, the alkoxide-treated alumina catalyst base is dried and calcined in a moist atmosphere, treated in an atmosphere of 50-100% relative humidity at 60.degree.-90.degree. F. (sufficient to form the metal oxide) and then calcined at between 800.degree. and 1,800.degree. F. The Group VIII metal is then impregnated into the support in the form of a salt or complex and the catalyst is dried at 150.degree.-300.degree. F. in the presence of nitrogen or oxygen or both, followed by calcination at 500.degree.-1,000.degree. F. The Group VIII metal impregnation and subsequent calcination may be conducted in the presence of oxygen or in an inert gas atmosphere.
U.S. Pat. No. 4,385,193-Bijwarrd et al relates to a process for the preparation of middle distillates by a conventional Fischer-Tropsch (FT) first stage and a hydro-reforming second stage. Suitable catalysts for the first stage are prepared by impregnation of a porous carrier, with one or more aqueous solutions of salts of FT metals and promoters. The promoters include Group IV B metals (e.g., zirconium) and the supports include alumina. A particular example is cobalt/zirconium/silicon oxide, prepared by impregnation of the silica with an aqueous solution of a cobalt and a zirconium salt, followed by drying, calcining at 500.degree. C. and reduction at 280.degree. C. No indication is given in the patent of any selectivity of the catalyst for the FT reaction.
U.S. Pat. Nos. 3,980,583, 4,393,225, and 4,400,561 to Mitchell et al claim catalysts for hydroformylation and the FT reaction, prepared from SiO.sub.2, Al.sub.2 O.sub.3, or an alumino-silicate, which may be combined with a Group IV metal oxide. The support is functionalized with an amine which binds the catalytic Group VIII metal to the surface.
In addition, it is otherwise well-known that ruthenium is useful as an FT catalyst.
Other recent literature, Basset, J.C.S. Chem. Comm., 1980, 154, has shown that catalyst prepared by the deposition of [Fe.sub.3 (CO).sub.12 ] onto an inorganic oxide support such as Al.sub.2 O.sub.3, MgO and La.sub.2 O.sub.3 followed by thermal decomposition results in the formation of a metal catalyst with a particle size distribution centered around 14 A.degree.. This catalyst is said to result in the selective synthesis of propylene from H.sub.2 and CO with propylene selectivities as high as 40% reported. Although the catalyst gives unusual selectivity to a fairly narrow range of products, its lifetime is short. After about six hours on stream, the catalyst begins to sinter and the selectivity drops off dramatically to give a more conventional Schulz-Flory distribution of hydrocarbons.
According to New Svntheses With Carbon Monoxide, J. Falbe (Springer-Verlag), 1980, metallic ruthenium will catalyze the formation of very high molecular weight hydrocarbons which are similar to polyethylene and are generally referred to as polymethylene. This process, however, only results in high molecular weights when the pressure is 1,000 to 2,000 atm. (103,000 to 206,000 kPa) and at temperatures of about 200.degree. C.
U.S. Pat. No. 2,632,014 discloses reacting carbon monoxide and hydrogen in a CO:H.sub.2 molar ratio of greater than 1:1, in the presence of water and a ruthenium catalyst, at a pressure within the range of 100 to 3,000 atm. and at a temperature of 150.degree. to 300.degree. C. Furthermore, the pH must be maintained below about 1. The catalyst may comprise a ruthenium-containing substance deposited on a support such as alumina.
For the synthesis of hydrocarbons from carbon monoxide and hydrogen, numerous patents (such as U.S. Pat. No. 2,284,468) disclose the use of a promoter such as an oxide of vanadium, cerium, iron, thorium, cobalt, chromium, barium, strontium, calcium, manganese, magnesium, zinc, lead, molybdenum, copper, zirconium or aluminum in combination with ruthenium catalysts.
U.S. Pat. No. 4,088,671 discloses the use of a ruthenium promoted cobalt catalyst in the synthesis of higher hydrocarbons from carbon monoxide and hydrogen In a low pressure synthesis gas process the catalyst is said to result in the substantial elimination of methane in the product with a simultaneous shift to the production of a higher carbon number product having a lower olefin content. The catalyst may be applied to a suitable support material such as alumina, boria, zinc oxide, magnesia, calcium oxide, strontium oide, barium oxide, titania, zirconia and vanadia.
U.S. Pat. No. 4,269,784 discloses a homogeneous process for preparing C.sub.9 to C.sub.60 hydrocarbons from carbon monoxide and water using a water-soluble ruthenium catalyst. The ruthenium catalyst may be a simple salt such as a halide, acetylacetonate or a complex salt of the formula [RuL.sub.6 ].sup.n where L is a neutral or charged ligand and n is the charge on the complex.
British Pat. No. 2,130,113 - Hoek, et al. (published May 31, 1984, priority date Nov. 22, 1982) discloses a Fischer-Tropsch catalyst with C.sub.3 -C.sub.5 selectivity. The catalyst uses a silica base, which is impregnated with an organic solution of, for example, zirconium or titanium propoxide and calcined. Cobalt is then deposited, as cobalt nitrate for example, from aqueous solution and is calcined.
U.S. Pat. Nos.4,413,064 and 4,493,905 Beuther, et al. (filing date Oct. 13, 1981, issue dates Nov. 1, 1983 and Jan. 15, 1985, respectively) disclose a Fischer-Tropsch catalyst, selective for the production of diesel fuel, based on a gamma or eta alumina support on which is deposited (a) cobalt and a Group IV B metal salt from a non-aqueous organic impregnant solution or (b) in a two-step process, preferably without calcination, cobalt (from an aqueous solution of, for example, cobalt nitrate) and then, from a non-aqueous organic solution, a ruthenium salt and a Group IV B metal oxide, the preferred oxides including ZrO.sub.2 and TiO.sub.2.
Notwithstanding the foregoing teachings, there remains a need for a selective, stable process and catalyst by which synthesis gas may be selectively converted to narrow carbon number range hydrocarbons and it is the general objective of the present invention to provide a stable catalyst and process effective to selectively produce such products, preferably at mild process conditions.